The entropy equation contains terms which indicate entropy generation due to two irreversible processes: Heat flow in the presence of temperature gradient, and gas flow in the presence of frictional force. These flows are assumed to be linearly dependent upon the temperature gradient and the frictional force. This assumption includes two cross phenomena: Convective heat transfer (set up by pressure gradient), and free convection (set up by temperature gradient). They are interdependent. Usually the frictional force is equal to the gaseous-phase pressure gradient. When this pressure gradient is zero, the heat flow depends only upon the thermal gradient. By entrapping the gas in the porous medium, the gas flow is stopped. This gives rise to a pressure gradient which sets up a convective heat flow opposing that due to thermal gradient. Consequently, the thermal conductivity of the porous insulator decreases. Experimental work on Tritex insulation material indicated a 22 per cent decrease in thermal conductivity.

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